The lifecycle of Arctic stratiform clouds is highly sensitive to precipitation rates, and in turn the fall velocity of snow particles. Frozen hydrometeor shapes are infinitely variable and their fallspeeds correspondingly difficult to predict. Many parameterizations for fallspeed have been developed as a function of hydrometeor habit, but based on mid-latitude precipitation. Even though snow clearly swirls, measurements or theory have contrived or assumed conditions for still air.
Recent Multi-Angle Snowflake Camera observations from the Arctic and elsewhere are helping to shed light on this problem by taking high-resolution photographs of hydrometeors in freefall while simultaneously measuring their fallspeed. Snowflake images collected at both Utah and Barrow show remarkably weak correlation between particle size or shape and fallspeed. However, particles fall at about 0.9 m/s on average in Utah, but 0.2 m/s on average in the Arctic, apparently due to a dependence of fallspeed on temperature through particle density. Moreover, the spread in measured velocities grows in proportion to ambient turbulence. The data suggest a need for revisions to commonly used parameterizations for precipitation in weather and climate models with the caveat that some fraction of MASC fallspeed measurements may be disturbed by interactions of ambient wind flow with the instrument itself.
